Huh? How would the shipping cost less unless the shipper charges less.
My point is that they do not
A more realistic outcome of cheaper gas prices is folks would be more willing to drive further to buy from brick and mortar rather than wait for shipping.
Here is a McKinsey Quarterly article with some details on Amazon’s supply chain:
https://www.mckinsey.com/~/media/mckinsey/dotcom/client_service/Retail/Articles/Future_of_retail_supply_chains.ashx
Amazon has an extensive freight network for transporting product from the warehouse to shipper distribution centers. Their costs from the shipper may not be tied to the fuel cost, but their own freight shipping costs are.